1. Field of the Invention
The present invention relates to a rolling bearing unit for a drive wheel of an automobile and a wheel driving unit.
The rolling bearing unit for a drive wheel of an automobile and the wheel driving unit according to the present invention are adapted to support a drive wheel to be supported by a suspension device of an independent type, which wheel includes the front wheel of an FF car (a front wheel drive car with a front engine), the rear wheel of an FR car (a rear wheel drive car with a front engine) and an RR car (a rear wheel drive car with a rear engine) and the all wheels of a 4WD car (a four wheel drive car), to be rotatable with respect to the suspension device, and are used for rotating and driving the above drive wheel.
2. Related Background Art
In order to support the wheel to be rotatable with respect to the suspension device, there are used various kinds of rolling bearing units for a drive wheel in which an outer ring or race and an inner ring or race are combined together to be rotatable through rolling elements. Also, a rolling bearing unit for a drive wheel which is adapted to support the drive wheel with the independent type suspension and to rotate and drive this drive wheel is required to have a structure which can be connected to a wheel side constant velocity joint. This wheel side constant velocity joint is required for transmitting a rotation of a transmission shaft for transmitting a drive force to the above drive wheel smoothly (maintaining uniform speed), irrespective of displacement of the drive wheel or a steering angle applied to this drive wheel. FIG. 7 shows a structure which is normally embodied according to the prior art, in which a rolling bearing unit 1 for the drive wheel and a wheel side constant velocity joint 2 are combined with each other for such a purpose.
In this structure, the rolling bearing unit 1 for the drive wheel comprises an outer ring 3, and a hub 4 and an inner ring 5 which are rotatably supported on the inner diameter side of the outer ring 3 through a plurality of rolling elements 6, 6. Out of these members, the outer ring 3 is connected and fixed to a knuckle (not shown) for constituting a suspension device by the use of a first flange 7 disposed on the outer peripheral surface thereof and is not rotated even when it is used. Also, on the inner peripheral surface of the outer ring 3, there are provided a pair of outer ring raceways 8, 8, so as to rotatably support the hub 4 and the inner ring 5 on the inner diameter side of this outer ring 3 coaxially with the outer ring 3.
Out of these members, the hub 4 is provided with a second flange 9 for supporting the wheel at a position closer to the outer edge of the outer peripheral surface. (The outer side with respect to or in the axial direction indicates the outer side in the width direction of the vehicle when it is assembled in the car and the left side in the drawings except FIG. 4. This will be the same in the entire present specification.) A first inner ring raceway 10 is formed in an intermediate inner ring element portion on the outer peripheral surface of the hub 4. The inner ring 5 which is formed with a second inner ring raceway 12 on the outer peripheral surface thereof is fitted and fixed on a small diameter stepped portion 11 corresponding to a fit cylindrical section formed at an axially inner end of the same outer peripheral surface. (The inner side with respect to or in the axial direction indicates the inner side in the width direction of the vehicle when it is assembled in the car and the right side in the drawings except FIG. 4. This will be the same in the entire present specification.) In addition, in the center of the hub 4, a spline hole 13 is provided.
On the other hand, the wheel side constant velocity joint 2 described above comprises a constant velocity joint outer ring 14, a constant velocity joint inner ring 15, a plurality of balls 16, 16, and a spline shaft 17. The constant velocity joint outer ring 14 is formed in the axially inner end portion of this spline shaft 17 coaxially with this spline shaft 17. Outer engagement grooves 18, 18 are formed at a plurality of positions in the circumferential direction of the inner peripheral surface of such a constant velocity joint outer ring 14, respectively in a direction perpendicular to this circumferential direction. The constant velocity joint inner ring 15 is provided with a second spline hole 19 in the center thereof and inner engagement grooves 20, 20 at positions which are corresponding to the outer engagement grooves 18, 18 on the outer peripheral surface thereof, respectively in a direction perpendicular to the circumferential direction. Then, the balls 16, 16 described above are disposed respectively between the inner engagement grooves 20, 20 and the outer engagement grooves 18, 18 to be rotatable along these engagement grooves 20, 18 in a state that they are held by a retainer 21. The shapes, and the like, of the constituent parts of the constant velocity joint 2 are the same as those of a known constant velocity joint of a Rzeppa type or a Birfield type.
The wheel side constant velocity joint 2 and the rolling bearing unit 1 for the drive wheel as described above are assembled to each other by inserting the spline shaft 17 into the spline hole 13 of the hub 4 mentioned above outward from inside thereof, and then bringing a nut 23 into threadable engagement with a male screw portion 22 which is formed in a part protruded from the outer end surface of the hub 4 at the axially outer end portion of the spline shaft 17 to be further tightened. In this state, the axially inner end surface of the inner ring 5 is contacted with the axially outer end surface of the constant velocity joint outer ring 14, so that this inner ring 5 is not displaced in a direction to be removed from the small diameter stepped portion 11 by any means. At the same time, an appropriate preload is applied onto the rolling elements 6, 6.
Further, in a state of being assembled in the suspension device of the car, a male spline portion 25 which is provided in the axially outer end portion of the transmission shaft 24 is brought into spline engagement with the second spline hole which is provided in the center of the constant velocity joint inner ring 15, so that this transmission shaft 24 is coupled with the constant velocity joint inner ring 15 to transmit a rotating force therebetween. Also, the axially inner end portion of this transmission shaft 24 is coupled and fixed to the central part of a trunnion 27 (see FIG. 12) which serves as an output unit of a constant velocity joint 26 of a tripod type which is provided in an output shaft portion of a differential gear. As a result, the transmission shaft 24 is rotated at a constant velocity when the car is running.
Disclosed in the Japanese Patent Application Laid-Open No. 11-5404 is a structure that an inner ring 5 which is fitted on the small diameter stepped portion 11 in the axially inner end portion of a hub 4a is pressed against a stepped surface 29 of the small diameter stepped portion 11 with a caulking portion 28 which is formed by caulking and expanding outward in the radial direction a cylindrical section existing in a part more protruded inward than the inner ring 5, as shown in FIG. 8. In case of this second example of the conventional structure, preload is applied onto each of the rolling elements 6, 6 with the pressing force exerted by the caulking portion 28. Note that, in case of the structure disclosed in the above laid-open application, like in the first example of the conventional structure described above, a rolling bearing unit 1a for the drive wheel and a wheel side constant velocity joint 2 are coupled to each other by bringing the male screw portion 22 which is provided in the outer end portion of the spline shaft 17 into threadable engagement and tight fitting with the nut 23. A similar structure is also disclosed in the Japanese Patent Application Laid-Open No. 2000-87979.
However, as shown in FIG. 8, in the structure that the caulking portion 28 is used to fix the inner ring 5 with respect to the hub 4a, the preload application onto the rolling elements 6, 6 is completed when this caulking portion 28 is formed. Accordingly, it is also possible to omit the male screw portion 22 and the nut 23 so as to reduce the size and the weight of the rolling bearing unit for driving the wheel which is comprised of a wheel driving rolling bearing unit and a wheel side constant velocity joint coupled to each other. FIG. 9 shows one example of a wheel driving rolling bearing unit 30, arranged from this angle.
In this wheel driving rolling bearing unit 30, it is arranged to prevent a spline shaft 17 which is inserted in a spline hole 13 formed in the center of the hub 4a from being drawn therefrom by the use of a stop ring 31 formed of an elastic material. To this end, an outer locking portion such as a locking stepped portion 32 is disposed in the axially outer end portion of the spline hole 13, and an inner locking portion such as a locking groove 33 is formed on the outer peripheral surface of the axially outer end portion of the spline shaft 17, respectively. Then, the stop ring 31 which is formed of a wire rod of spring steel in a split ring shape with a split at a portion thereof and whose diameter is elastically expandable or contractible is extended over between the locking stepped unit 32 and the locking groove 33. As described above, it is possible to reduce the size and the weight of the rolling bearing unit 30 for driving the wheel by using the structure in which the spline shaft 17 is prevented from being drawn out from the spline hole 13 by the stop ring 31 and a rolling bearing unit 1b for the drive wheel is coupled to the wheel side constant velocity joint 2 to each other, so that the male screw portion and the nut may be omitted.
Also disclosed in U.S. Pat. No. 5,490,732 is a structure that the inner ring 5a which is fitted on the small diameter stepped portion in the axially inner end portion of a hub 4c is pressed against a stepped surface of the small diameter stepped portion by the use of the caulking portion 28 which is formed by caulking and expanding outward in the radial direction a cylindrical section existing in a part more protruded inward than the inner ring 5a, as shown in FIG. 10. In case of this disclosed structure, the hardness of the caulking and the dimensions of each constituent parts to allow the caulking work are specified. Note that, in the above patent publication, there is also disclosed a drawing in which a boarder position between a thicker cylindrical section and a thinner cylindrical section on the axially inner end portion of the hub 4c is positioned more outward in the axial direction than the axially outer end position of a chamfered portion which is formed on the inner ring fixed to the hub at a point of intersection between the inner peripheral surface with the axially inner end surface suppressed by the caulking portion. However, in this patent publication, a structure having a spline at the center of the hub is not disclosed and the dimensions of each of the constituent parts of the hub for minimizing the contraction of a female spline at the axially inner end of the hub due to the caulking are not specified.
Also, disclosed in European Patent No. 1063437 is a structure that the inner ring 5 which is fitted on the small diameter stepped portion on the axially inner end portion of a hub 4d is pressed against the stepped surface of the small diameter stepped portion by the use of a caulking portion 28 which is formed by caulking and expanding outward in the radial direction a cylindrical section existing in a part more protruded outward than the inner ring 5, as shown in FIG. 11. In this publication, the position at which the caulking work of the hub 4d starts is located more outward in the axial direction than the axially outer end position of the chamfered portion which is formed at a point of intersection between the inner peripheral surface of the inner ring 5 fixed to the hub 4b, with the axially inner end surface of the inner ring 5 suppressed by the caulking portion 28, so as to control the axial force to be applied on the hub 4d due to the caulking. However, in this publication, the dimensions of each of the constituent parts of the hub for minimizing the contraction of a female spline at the inner end of the hub due to the caulking work are not specified. Note that in this publication, the thickness of the spline portion is not specified at all. Also in this publication, the position at which the caulking work starts is defined. However, this position is on the boarder position between a thick part and a thin part of the hub, and has the same structure as that disclosed in the above U.S. Pat. No. 540,732.
The rolling bearing unit 30 for driving the wheel as described above is combined with the transmission shaft 24 and the differential side constant velocity joint 26 to constitute a driving unit 34 for a wheel, as shown in FIG. 12. Out of these members, the differential side constant velocity joint 26 is coupled to an output portion of an unrepresented differential gear in a state that it is assembled in the car. Of the transmission shaft 24, the axially inner end portion is coupled to the basic end portion of the trunnion 27 which serves as an output portion of the differential side constant velocity joint 26, while the axially outer end portion is coupled to the constant velocity joint inner ring 15 which serves as an input portion of the wheel side constant velocity joint 2, respectively. The rotating force is transmitted by such a driving unit 34 for the wheel from the output portion of the differential gear to the drive wheel supported by the hub 4b, thereby rotating and driving this drive wheel.
In case of such a structure as shown in FIG. 8 to FIG. 12 above in which the inner ring 5 is fixed with respect to the hubs 4a to 4d by the caulking portion 28, the cost can be reduced by omitting the male screw portion 22 and the nut 23 from the structure shown in FIG. 7. Moreover, in this case, the size and the weight of the wheel driving rolling bearing unit 30, and consequently, those of the wheel driving unit 34 can also be reduced. Out of these members, the wheel driving rolling bearing unit 30 is provided closer to the wheel than a spring assembled in the suspension device to render a so-called unsprung load, so that a slight reduction of the weight can contribute to enhancement of the running performance centering on a riding comfort or running stability.
It is required to sufficiently maintain the strength of the caulking portion 28 for fixing the inner ring 5 to the hubs 4a to 4d for constituting the rolling bearing units 1a, 1b for the drive wheel, which is to be used for a long time. Accordingly, a load required for forming this caulking portion 28 becomes considerably large even when such a method is employed in which a load to be applied during the processings such as a rocking caulking can be suppressed to be comparatively low. Thus, in this case, such load is applied as a thrust load which is directed outward in the axial direction onto the inner end portions of the hubs 4a to 4d. As a result, according to the experiments conducted by the present inventor, it has been found that, if no counter measure is taken, a part of the spline hole 13 (particularly, a part closer to the axially inner end) formed at the center of each of the hubs 4a to 4d is deformed, though slightly, inward in the radial direction.
More specifically, hitherto, when the cylindrical section provided in the axially inner end portion of each of the hubs 4a to 4d is processed to be the caulking portion 28, since this cylindrical section is caulked and expanded outward in the radial direction, the axially inner end of the spline hole 13 which is positioned in the vicinity of this caulking portion 28 is assumed to be deformed outward in the radial direction. However, the thrust load applied during the processing of the caulking portion 28 is large, as described above, so that an influence of a lateral distortion on this thrust load becomes larger than conventionally thought. As a result, the axially inner end portion of the spline hole 13 which is positioned in the vicinity of the caulking portion 28 is deformed inward in the radial direction, conversely, according to the findings of the present inventor. In any case, when even a part of the spline hole 13 is deformed inward in the radial direction, it becomes difficult to insert the spline shaft 17 into the spline hole 13, so that an assembling work of the rolling bearing unit 30 for driving the wheel is difficult to be conducted.
In order to facilitate the insertion of the spline shaft 17 into the spline hole 13 in spite of the deformation of the spline hole 13, it can be considered to sufficiently expand the inner diameter of this spline hole 13 to be larger than the outer diameter of the spline shaft 17, or to process the spline hole 13 for the second time after the processing of the caulking portion 28. Out of them, expansion of the inner diameter of the spline hole 13 may result in an increase of a backlash in a spline engagement portion between this spline hole 13 and the spline shaft 17, which may undesirably cause a conspicuous foreign sound or noise during when the automobile is driven. In addition, re-processing of the spline hole 13 may cause an increase of the manufacturing cost of the rolling bearing units 1a, 1b for the drive wheel, as another undesirable effect.